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Light, found to consist of a rectangular border enclosing no less than nine several compartments, arranged as in Part IVi fig. 223, and separated from each other, and from the border, by delicate lines or films as there marked. Each - v - 1

of these compartments possesses its own peculiar crystallographic structure, and polarizes its peculiar tints, thelaw of symmetry being observed. In some specimens the triangular spaces p q r s were wanting, while in othersthey seem to have consisted of two portions, separated by an imaginary prolongation of the line joining theirobtuse angles with the central lozenge.

The terminal plates, the ceutral lozenge, and the minute stripes dividing the compartments from each other(which are sections of laminae or films parallel to the axis of the crystal, and running its whole length) consistof that uniaxal variety, in speaking of which we have used the term leucocyclite, from the whiteness of its rings.

The rectangles R V , S T, (with the exception of the portions occupied by the lozenge and partitions) consist ofa biaxal medium, having its axes 34° inclined to each other, and its optic meridian parallel to the axis of theprism, and passing through the diagonals R V , S T of these rectangles. The other rectangles are composedof a similar medium, but with its optic meridian at right angles to the former, or passing through the diagonalsR T, S V.

A still more remarkable and artificial structure has been observed by Dr. Brewster, in a variety of the Faroe 1131.apophyllites of a greenish white hue. When a complete prism of this variety is exposed to polarized light, with Anotherits axis in 45° of azimuth, the light being transmitted perpendicularly through two opposite sides, the pattern varlet y-represented in fig. 224 is seen, in which the central curvilinear area is red, and its complements to the surround- F ‘S' 224.ing rectangle green. The squares immediately adjacent on either side in the direction of the axis are also vividred in their centres, fading into white, while the rest of the pattern consists in a most brilliant succession of red,green, and yellow, bands, for a coloured figure of which we must refer the reader to the original most curiousand interesting memoir, ( Edinburgh Transactions, vol. ix. part ii.) where, as also in the Edinburgh PhilosophicalJournal, vol. i. he will find the phenomena described in full detail.

The sulphate of potash oilers another very remarkable example of compound structure. This salt occurs in 1132.hexagonal prisms, and occasionally in bipyramidal dodecahedrons. But besides these forms it also occurs in Sulphate ofrhombic prisms of 114° and 66°. These Dr. Brewster found to have two axes, while the hexagonal prisms have P 0,ash -but one; thus affording another instance of dimorphism in addition to those of arragonite, sulphur, &c. On ex-amining the dodecahedrons, however, he found them to consist of six equilateral trianjrular prisms, of the biaxalvariety, grouped together, and having their optic meridians all converging to the common axis; the moleculesbeing so disposed in each opposite pair of individuals as to make the angle between the opposite faces of eitherpyramid (114°) equal to the obtuse angle of the rhomboid.

The structure and mode of action of the analcime, described by Dr. Brewster in vol. x. of the Edinburgh Trans- 1133.actions, part i. p. 187, are so extremely singular, that it is difficult to say whether it should be regarded as a Analcime.grouped crystal, consisting of independent portions adhering together, or as amass the distribution of the ether inwhose parts is governed by a general and uniform law ; the latter, however, is probably the truth. The form ofthis crystal is the icositetrahedron, contained by twenty-four similar and equal trapezia, and may be regarded asderived from the cube by the truncation of each of its angles by three planes symmetrically related to the edgesincluding it. If we conceive from the centre of this cube, (in its natural situation with respect to the derivedfigure) planes to pass through each of the edges, and through each of the diagonals of the six faces, they willdivide the cube into twenty-four irregular tetrahedra; and of these, all the faces which pass through edges of thecube will also pass through edges of the derived figure, while those which pass through diagonals of faces of thecube will also pass through diagonals of the faces of its derivative, bisecting their obtuse angles. Now it appearsfrom Dr. Brewster’s observations, that all the molecules situated in any part of anyone of these planes are devoidof the power of double refraction and polarization ; and that in proportion as a molecule is distant from all suchplanes, its polarizing power is greater. In this respect it differs entirely from all crystals hitherto examined,every particle of which, wherever situated, so long as they belong to one and the same crystalline system, beingequally endued with the polarizing virtue. Nor is there a closer analogy between the mode of action in question,and that of unannealed glass and similar bodies ; for in these a change of external form is always accompanied witha change of the polarizing powers, while in the analcime each particular portion, whether detached from the mass,or in its natural connection with the adjacent molecules, possess the very same optical properties. The actiontoo of the portions which possess a polarizing power is not related to axes given only in direction, and passingthrough every molecule, but to planes given both in direction and in place within the mass, (the planes abovementioned ;) the tint developed at any point of a plate being as the square of the distance from the nearest of suchplanes, and the isochromatic lines being, in consequence, straight fringes of colour arranged parallel to the darkbands marked out by the intersection of such planes with the plate examined. The phenomena described areaccompanied with a sensible double refraction. The reader is referred to the memoir already cited (which is oneof the mcv«t interesting to which we can direct his attention) for further details: and to a work understood to beforthcoming from the pen of the eminent author here and so often before cited, on optical mineralogy, for whatwe are sure wiU prove a treasure of valuable information on every point connected with this important applicationof optical science.

§ XIV. On the Colours of Natural Bodies.

It was our intention to have devoted a considerable share of these pages to the explanation of such naturalphenomena as depend on optical principles, but the great length to which this essay has already extended, rendersit necessary to confine what we have to say on such subjects within very narrow limits, and to points of promi-

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